This invention relates to a method of drawing a metal foil or sheet, or plastic film or sheet, or their laminate into formed articles, and equipment for practicing same and formed articles manufactured by the method. The present invention is directed more particularly to a method of drawing a metal foil or sheet, or a plastic film or sheet, or their laminate which is of the order of 7 to 200 .mu.m in thickness and has a relatively low rigidity, into a formed article which has no wrinkles, and equipment for practicing same and formed articles manufactured by the method.
Containers extremely thin in wall thickness which are made of a metal foil or sheet such as aluminum foil or tin plate, or a plastic film or sheet, or their laminates, are extensively employed for packaging goods or medicines and so on. However, such containers have scarcely been formed by drawing. This is due to the following reasons: These foil, sheet, film and laminate, namely, blanks are less in rigidity. Accordingly, if the blanks are drawn by the ordinary drawing method, wrinkles are significantly created on the formed article or container or shell at the radiused corner of the die and are distributed substantially entirely over the flange and the sidewall portion of the shell. This will undoubtedly defile the appearance of the containers. In addition, it is impossible to hermetically seal the containers with their flange portions by lids so as to prevent leakage of the contents and deterioration of the contents due to entry of air and moisture.
In order to overcome this difficulty, a method is extensively employed in which, after heat-softened, a plastic film or sheet, or a composite film or sheet consisting of plastic materials is subjected to pressurized air forming or vacuum forming to produce a formed article. This method is a so-called bulging method which can be applied only to plastic films or sheets which extend 100% to 200% or more at heat-softened state. Accordingly, this method is disadvantageous in that a step of heating is required, and unlike drawing, it is impossible to subject the blank to cold forming which is carried out at room temperature or a temperature around the room temperature. It goes without saying that this method cannot be applicable to aluminum foil or tin foil.
In the case where containers are made of plastic only, it has been found that it is rather difficult to attain complete barrier for oxygen or moisture, that is, such containers are not suitable in the field of packaging foods to be preserved for a long time at room temperature where sophisticated technique is required. In order to eliminate this drawback, a laminate of aluminum foil, or the like, and plastic films or sheets has been provided recently. In this case, in view of material saving and cost reduction, the aluminum foil should be as thin as possible, but to the extent that the contents in the container is prevented from inroading of oxygen and moisture. Accordingly, laminates made of a thin aluminum foil 7 to 30 .mu.m in thickness and polyester, polyethylene, polypropylene, nylon or polycarbonate films etc. several to several tens of .mu.m in thickness have been provided to package foods or the like. However, since the laminate of this type contains aluminum foil, the elongation thereof is no more than several to 50%, and therefore manufacturing a formed article from the laminate in accordance with the above-described pressurized air or vacuum forming is practically impossible. Accordingly, such laminates are used mainly for manufacturing pouches.
In a drawing method, unlike a bulging method, the thickness of a blank sheet is scarcely changed during drawing, and the elongation of the blank needs not to be so high. Accordingly, in the drawing method, it is possible to obtain a formed article such as a cup by subjecting the aforementioned very thin metal foil or sheet, or a plastic film or sheet or laminates thereof to cold-drawing. However, in this conventional drawing, wrinkles are remarkably created on the formed article at the radiused corner of the die, and therefore heretofore it is impossible to manufacture formed articles which can be commercialized.
In other words, in the conventional drawing, when a blank is drawn into the die by the punch, the blank has a portion which is not restrained on the radiused corner of the die. However, in the case where the blank is sufficiently thicker than 200 .mu.m and especially it is a metal sheet, the rigidity of the blank is considerably high, so that wrinkles are scarcely created on the formed article at the radiused corner. On the contrary, the rigidity of a plastic laminate thinner than 200 .mu.m including a thin aluminum foil 7 to 30 .mu.m is relatively low, and therefore if this plastic laminate is drawn in accordance with the conventional method, wrinkles will be necessarily created.